Automotive electrical wire located under the hood in the engine compartment has traditionally been insulated with a single layer of high temperature insulation disposed over an uncoated copper conductor. Thermoplastic polyesters, cross-linked polyethylene and halogenated resins such as polyvinyl chloride have long filled the need for the high temperature insulation needed in this challenging environment that requires heat resistance, chemical resistance, flame retardance, and flexibility.
Thermoplastic polyester insulation layers, with outstanding resistance to gas and oil, are mechanically tough and resistant to copper catalyzed degradation but can fail prematurely due to hydrolysis. The insulation layers in thermoplastic polyester insulated electrical wires have also been found to crack when exposed to hot salty water and have failed when subjected to humidity temperature cycling.
There is an increasing desire to reduce or eliminate the use of halogenated resins in coverings. In fact, many countries are beginning to mandate a decrease in the use of halogenated materials. However, as much of the wire coating extrusion equipment was created based upon the specifications of halogenated resins such as polyvinyl chloride, it is desired that replacement materials be capable of being processed in manners similar to polyvinyl chloride.
Cross-linked polyethylene has largely been successful in providing high temperature insulation but this success may be difficult to sustain as the requirements for automotive electrical wire evolve. The amount of wiring in automobiles has increased exponentially, as more electronics are being used in modern vehicles. The dramatic increase in wiring has motivated automobile manufacturers to reduce overall wire diameter by specifying reduced insulation layer thicknesses and specifying smaller conductor sizes. For example, ISO 6722 specifies, for a conductor having a cross sectional area of 2.5 square millimeters, that the thin wall insulation thickness be 0.35 millimeters and the ultra thin wall insulation thickness be 0.25 millimeters.
The reductions in insulation wall thickness pose difficulties when using crosslinked polyethylene. For crosslinked polyethylene the thinner insulation layer thickness results in shorter thermal life, when aged at oven temperatures between 150° C. and 180° C. This limits their thermal rating. For example, an electrical wire having a copper conductor with an adjacent crosslinked polyethylene insulation layer having a 0.75 millimeter wall thickness is flexible and the insulation layer does not crack when bent around a mandrel after being exposed to 150° C. for 3,000 hours. But a similar electrical wire having a crosslinked polyethylene insulation layer with a 0.25 millimeter wall thickness the insulation layer becomes brittle after being exposed to 150° C. for 3,000 hours. The deleterious effects created by these extremely thin wall requirements have been attributed to copper catalyzed degradation, which is widely recognized as a problem in the industry.
It is possible to coat the copper core with, e.g., tin in order to prevent the copper from contacting the crosslinked polyethylene but the additional cost of the coating material and the coating process are expensive. In addition, many automotive specifications require that the copper conductor be uncoated. It is also possible to add stabilizers, also known as metal deactivators, to the insulation material but it is recognized that stabilizers yield only partial protection for electrical wire having thin wall thicknesses.
It has been proposed to employ bilayer or trilayer insulation materials wherein a protective resin based layer is disposed between the crosslinked polyethylene and the copper conductor. However, manufacture of bilayer and trilayer insulation materials is complex, requires increased capital expenditure and the multilayer material presents new issues of inter layer adhesion.
Accordingly, there is an ongoing need for electrical wires having a halogen free covering that are useful in the automotive environment.